The invention is located in the field of plasma torches.
Arc plasmas belong to the family of thermic plasmas. They are partially ionised gaseous media, conductive of electricity but by and large electrically neutral, at pressures in the region of atmospheric pressure. They are generated by means of a plasma torch, by passing one or more plasmagene gases through an electric arc which is maintained between two electrodes.
To bring gases to a high temperature and high specific enthalpy, blown arc torches are used. This means that the arc is confined to the inside of the torch containing the two electrodes and it is the high speed jet of high temperature gas (the plasma) which is used in the process.
FIG. 1 shows in a very diagrammatic way the principle of such a torch. A torch of this kind includes two electrodes, an anode 1 and a cathode 3, concentric with each other and providing between them a gas circulation channel 7.
The two electrodes 1,3 are connected to a high voltage, high frequency (HV-HF) generator and to a direct current generator. They must of necessity be energy cooled (by water circulation) to prevent their fusion.
Initially and by means of the HV-HF generator, an electric arc 8 flashes between the two electrodes (cathode and anode) ionising the gas introduced and making the inter electrode space conductive. The direct current generator may then issue into this space and maintain the arc.
The power supplied to the torch is equal to the product of the strength (which can be regulated) of the voltage established between the anode and the cathode. This voltage is dependent on several parameters such as the type and flow of gas used, but also, to a not insignificant degree, on the wear and tear on the electrodes. The power of the plasma 9 is equal to the power supplied to the torch minus the losses in the cooling water. Wear and tear on the electrodes puts them therefore at a serious disadvantage. It depends on their geometry, their cooling efficiency, their coaxiality, and on the type and purity of the gases.
Equipment allowing an arc 8 plasma 9 to be generated is used for thermal spraying (surface treatment), gas heating or chemical synthesis. The energy supplied to the gas(es) by the electric arc allows them to be heated to temperatures above 10,000 K.
The choice of plasmagene gas or gases is almost unlimited. It is a function of the demands of the process (oxidation, nitridation, high temperature in a reduction medium, etc.). The power range is very extensive, running from a few kilowatts to several megawatts. Very often, the potential operational range is dictated by the type and flow of the plasmagene gases selected.
A torch is therefore often designed for a given application since its technology must be compatible with the choice of plasmagene gas and the desired work power.
Its size, its form and its simplicity can also become important if it is required to work in a cramped or hostile environment.
Torches currently in existence are complex units, including at least about ten parts (excluding seals, screws and fluid connectors). The coaxiality of the electrodes depends on the stack of manufactured parts with acceptable tolerances for the seals.
Replacing one or both electrodes is an operation which has to be performed regularly (in most cases after some ten hours in operation). This operation always requires sub-units to be dismantled/re-assembled and the seals to be changed.
To illustrate this, three examples of known plasma torches will now be briefly described.
A first known torch operates with an air/argon or oxygen/argon mix, its power is about 100 kW. It consists of 15 manufactured parts, 21 seals, 22 screws and 6 fluid connectors. The parts subject to regular wear are the cathode and the anode, an insulation bush and an injection nozzle. Minimum maintenance (changing the anode) is required at less than 100 hours of operation in the best conditions of use.
A second known torch has been developed for the hydropyrolysis of heavy hydrocarbons. The plasmagene gases are argon and hydrogen, which are mixed with methane at the torch output. This torch is similar to a thermal spray gun. It has, excluding the fluid feed connectors and the screws, 10 manufactured parts and 7 O-rings.
As a third example may be cited one of the simplest torches, marketed by the company SULZER METCO. This is the thermal spray gun F4-MB. This type of torch operates conventionally with argon, helium and nitrogen singly or in a mixture. Hydrogen is often added to gain power (increase in peak arc voltage). There are nevertheless 8 manufactured parts, 14 O-rings, 12 screw components and 3 fluid connectors.
Japanese patent application JP 04-249 096 describes a plasma torch wherein, in order to reduce the probability of creating an arc between the anode and the cathode, the plasmagene gases follow a path which allows them to eddy. To this end, a centring device 10a, which is a part placed between the anode and the cathode, has an opening 106, which goes from the top face of the centring device to a lateral face. Another conduit 102 located between the anode and the centring device part 10 allows the gases coming from the conduit 106 to be guided to the bottom of the anode.
Conduits 107 join the outside of the centring device 10 to a central cavity 105 of the latter. This particularity allows an eddying jet of plasmagene gas to be created. A more even wear of the cathode is thus obtained.
Document EP-0 002 623 A describes a torch wherein the cathode 13 is mounted on a support 23, mobile through rotation of a nut 26 in an axial direction. It is thus possible to adjust finely the value of the stoke hole between the cathode 13 and the anode 14.
The invention described in this document relates essentially to means for imparting to the electric arc between anode and cathode an even rotation so as to prevent rapid wear of the electrode by the arc locking for too long at given points. In the example shown in FIG. 1 or 11, it is a magnet 19 (FIGS. 1-11) generating a magnetic field B (FIG. 4) parallel to the axial direction of the torch.
According to one configuration (FIG. 8), it is a tangential inflow of gas 32xe2x80x2 into a flow regulation chamber 15.
According to one shown configuration (FIG. 11), the means is constituted of a helical flap 40 placed on the gas path between the chamber 15 and the nozzle 38.
Cooling is provided by water circulating in conduits 20-21 (FIGS. 1, 8, 11).
The purpose of the torch, according to the invention, is as far as possible to simplify the assembly of the torch itself and, on the other hand, the replacement from time to time of worn out parts. It has been developed in particular for a gas heating application in a gas postcombustion reactor for pyrolysis of chlorinated radioactive waste, heavily contaminated by alpha emitters. This reactor is intended to operate in a glove box.
In a hostile environment (radioactive, being compelled to work in a glove box or in the remote manipulator), the work must be simplified as far as possible. The standard exchange of sub-units is often preferable to the dismantling and to the re-assembly of isolated parts in a complex unit. The intervention time is shorter, the reliability of a new and inspected sub-unit is much better than that of a dismantled and re-assembled complex unit.
To this end, the plasma torch according to the invention is designed in two parts, a disposable interchangeable cartridge constituting a plasma generator intended to be inserted into a cartridge connecting and holding structure. The purpose of this cartridge connecting and holding structure is to connect the cartridge to its supplies of plasmagene gas, cooling fluid and electrical currents. This structure comprises to this end first cartridge connection means.
These first means serve as intermediaries for the supplies of electrical currents, water and gas. These supplies are therefore completely dissociated from the plasma cartridge.
The structure comprises second means engaging or not engaging with cartridge fixing means so as to keep the cartridge mechanically connected to the first means of supplying electrical currents, water and gas.
The invention relates to a cartridge generating plasma for a plasma torch, having the characteristics featured in claim 1.
In this way the mechanical assembly is made with a single auxiliary assembly part, the assembler, by simple operations carried out under pressure to push the projecting parts along an axial direction into the hollow parts. It will be seen later that with adapted tools a single pressure operation is necessary.
In the preferred embodiment, the projecting parts comprise a first and/or a second annular ring, which fixes itself in a first and/or second annular groove. As a result of this shape, which offers a symmetry of revolution, assembly is simplified, since the parts to be assembled have only to be coaxially positioned with the assembler, the anode and/or the cathode support, for fixing to be completed, without it being necessary to index them angularly.